PROJECT SUMMARY Virtually every cancer that takes the life of a patient is due to innate or acquired chemoresistance. This is especially true in epithelial ovarian cancer (EOC), in which most tumors are initially sensitive to platinum-based chemotherapy, but most will recur and succumb to chemoresistant disease. To achieve durable cures we must understand the molecular mechanisms of chemoresistance. Through in-depth analysis of multiple models of matched pre- and post-chemotherapy (carboplatin/paclitaxel) ovarian cancers from treated patients, patient- derived xenografts (PDX), and resistant cell lines, we have discovered and validated that chemoresistant tumors have significant upregulation of the ribosomal biogenesis pathway. We have further examined efficacy of two inhibitors of RNA Polymerase I (Pol I), the primary regulator of rRNA production. These agents, CX-5461 and BMH-21, have significant (but frequently variable) activity against ovarian cancer cell lines and PDX models of all histologies, and in many cases is even more effective in chemoresistant models. CX-5461 is currently in a phase I trial, but we are the first to demonstrate and explore the particular susceptibility of chemoresistant cells to targeting ribosomal biogenesis, and why this process might be key to developing chemoresistance. Several questions remain unanswered, including whether targeting Pol I can kill the post-chemo microscopic remaining population to achieve durable cures; how upregulation of ribosomal machinery enhances chemoresistance; what transcriptome is activated by chemotherapy; whether the effects are specific to paclitaxel, carboplatin, or the combination; and whether the hypothesized critical role of TP53 in the efficacy of these agents can allow strategies to allow targeting Pol I to be even more effective. The overall objectives of this proposal are to understand how upregulation of ribosome biogenesis allows cancer cells to survive chemotherapy, identify the most effective setting in which to target Pol I as a therapy, and identify the best agents to use in combination with Pol I for therapeutic synergy. To achieve these objectives, we will investigate in greater detail the chemotherapy-induced differences in ribosome synthesis between the chemosensitive and chemoresistant cell populations using multiple models, and identify how these differences are mediating Pol I inhibitor sensitivity. Chemoresistant PDX models will be used to determine if Pol I targeting can prevent recurrence, or enhance carbo/paclitaxel efficacy. We will investigate the differences between chemosensitive and chemoresistant cells at the level of chromatin structure, occupancy of rRNA DNA transcription sites, and ribosomal organization. We will utilize a 7,000-gene CRISPR library of druggable targets to identify candidate drugs to use in combination with targeting Pol I. If the role of ribosomal biogenesis in chemoresistant cells can be better understood, it could open the door to an entirely new approach to treating many cancers, and focus on the most deadly aspect of cancer ? evolution to a chemoresistant phenotype for which there is no cure.